1. Field of the Invention
The present invention relates to an image recording apparatus which records and reproduces an image by performing a scan on a photoreceptor with a light beam which is modulated based on an image signal. More specifically, the present invention relates to an image recording apparatus which simultaneously records a plurality of line images by scanning and exposing the photoreceptor with a plurality of laser beams.
2. Description of the Background Art
In the market of image recording apparatuses such as a laser printer having a laser recording apparatus or a digital copying machine, the conventional machines which have been introduced into the market range from the low-cost and low-speed machines to the high-cost and high-speed machines. In the market of image recording apparatuses operating at high recording speed, all high-level functions which are not only high-speed output but high image quality are required relative to all specifications.
Regarding the laser recording apparatus, the high-speed output could be achieved by increasing the number of revolutions of a photoreceptor and enhancing the scanning speed of a laser scanning device so as to record an image on the photoreceptor rotating at high speed.
In order to enhance the scanning speed of the laser scanning device, a rotational polygon mirror should be rotated at high speed for scanning with the laser beam in a predetermined direction (main scanning direction).
However, if the method of enhancing the speed of rotational polygon mirror is employed, a motor for the rotational polygon mirror inevitably increases in size, so that a problem of radiating heat from the motor arises. At present, the size of the entire apparatus and the space where the apparatus is to be installed should be considered, and the problems such as the noise due to enhanced speed and increase in weight of the apparatus have arisen. Therefore, the extent of enlargement of the motor is limited.
In the high speed laser recording apparatuses of today, the enhancement of the speed is achieved by not only speeding up the rotation of the rotational polygon mirror but simultaneously recording a plurality of line images by one scanning operation with a plurality of laser beams.
However, when a plurality of laser beams are used to simultaneously record a plurality of line images, the points at which a scan is started on the plurality of line images on the photoreceptor could deviate from each other (jitter) in the main scanning direction. This is due to displacement of positions where a plurality of laser sources are arranged in parallel with each other.
A method is now known for recording and reproducing an image with high resolution by inclining a plurality of laser sources and arranging them in parallel with each other, so as to decrease the pitch of the line images in the sub-scanning direction and accordingly achieve continuous gradations. In this case, the plurality of inclined laser sources start scan at points deviated from each other in the main-scanning direction.
When the positions where the exposure and scan are started by the laser beams are thus slightly displaced on the photoreceptor (the scan starting points are not aligned in the main-scanning direction), it is impossible to achieve fine gradations of an image which is recorded and reproduced.
A method is accordingly devised for aligning the scan starting points in the sub-scanning direction on the photoreceptor, with which each laser beam is detected before starting the exposure and scan of the photoreceptor, and after a predetermined time set for each laser source has passed (after counting reference clock a predetermined number of times), each laser source is driven (turned on) with modulation based on image data.
In order to implement this method, conventionally a plurality of image clock signals having different phases are generated in advance, and an image clock signal which satisfies a predetermined condition is determined when each laser beam is detected before starting exposure and scan of the photoreceptor. The laser source is then driven with modulation by the determined image clock signal.
The structure mentioned above is disclosed in U.S. Pat. No. 4,393,387 and Japanese Patent Laying-Open Nos. 60-153259 and 9-1859.
U.S. Pat. No. 4,393,387 discloses a recording apparatus capable of correctly matching image writing starting positions with a simple structure by inclining a plurality of laser beams and synchronizing a plurality of clock signals that correspond to respective beams based on the result of detection by a beam detector.
Specifically, a plurality of laser beams are inclined, and one position signal is generated based on the result of detection by the detector which detects a particular beam which is turned on. Based on the position signal, a plurality of clock signals having the same period and different phases are produced. The plurality of clock signals are applied to a plurality of memories which store recording signals, and recording signals corresponding to respective beams are read out.
Japanese Patent Laying-Open No. 60-153259 discloses a synchronizing device of a laser printer capable of decreasing the jitter by providing a clock delay unit which successively delays reference clocks with a constant phase and selecting one of thus produced clocks as an image clock which maintains a constant relation with an output of a photodetector.
Specifically, when laser beam reaches the photodetector, a photo-detection output pulse HS is output. The photo-detection output pulse HS is supplied to a latch circuit, and a combination of a plurality of clocks T0 and T1 is supplied to a data input of a data selector. After clocks T0 and T1 are respectively delayed by xc2xc period to generate pulses T2 and T3, any one of pulses T0 to T3 is selected. Pulse data supplied to select inputs SA and SB of the data selector are synchronized by the photo-detection output pulse HS. Therefore, the pulse data selected by the data selector and output from an output terminal Y is substantially synchronous with pulse HS. The output is accordingly used as an image clock so as to implement image recording with reduced jitter.
Japanese Patent Laying-Open No. 9-1859 discloses an image forming apparatus capable of synchronizing a synchronizing signal with an image clock precisely without using a high-frequency reference clock and without using a number of clocks having different phases, by utilizing an analog periodic signal.
Specifically, a synchronizing signal indicating a starting timing of image formation is generated for each line in the main-scanning direction. According to the timing at which the synchronizing signal is generated, the voltage value of the analog periodic signal with periodically changing voltage value is sampled and held. Thus, the timing at which the synchronizing signal is generated relative to a reference clock signal is converted to the voltage value to be stored. Comparison between the analog periodic signal and the stored voltage value allows a pixel clock to be produced with a phase corresponding to the timing at which the synchronizing signal is generated.
According to the methods disclosed in U.S. Pat. No. 4,393,387 and Japanese Patent Laying-Open No. 60-153259, however, a plurality of image clock signals having different phases respectively should be generated in advance. In order to minimize the displacement in the recording starting position in the main-scanning direction, the phases of the image clock signals should be displaced finely from each other so as to generate a large number of image clock signals. As a result, the number of delay circuits for displacing the phases increases, leading to a complicated circuit structure.
Consequently, the clock frequency of a plurality of image clock signals increases, making it difficult to implement the circuit, or may cause problems such as high-frequency noise and increase in cost due to employment of a high-speed element.
Further, the delay circuits used for generating a large number of clocks that are different from each other in delay time could not ensure a stable delay amount in the event temperature or voltage varies.
Even if a plurality of image clock signals having different phases are generated, it cannot be guaranteed that an image clock is obtained which satisfies a predetermined condition when a laser beam is detected before start of exposure and scan of the photoreceptor. Therefore, there arises a problem in eliminating the displacement of positions where exposure is started by a plurality of laser sources.
In the case of the art disclosed in Japanese Patent Laying-Open No. 9-1859 which employs the analog periodic signal, a sample-hold circuit and differentiator circuit are required, leading to an increase in throughput of the circuit and difficulty in realizing short-time processing. In particular, if the circuit uses a capacitor, the accuracy of the element is generally inferior to other elements causing a significant unevenness of products. As a result, available products must be selected and accordingly, cost is increased.
In addition, the, analog periodic signal is used as a reference signal. Therefore, influence by variation of the power supply or unevenness of elements causes a sawtooth waveform. Compared with a digital signal, the analog periodic signal is liable to cause disadvantages such as unevenness of image clocks and variation in the scan starting position.
The present invention is envisioned to solve the above-noted problems . One object of the present invention is to closely reproduce, using a simple structure, variation in intensity of an image which is recorded on a photoreceptor so as to obtain an even image with superior gradations.
Another object of the present invention is to closely reproduce, without difficult positional adjustment, variation in intensity of an image which is recorded on a photoreceptor so as to obtain an even image with superior gradations.
According to one aspect of the invention, an image recording apparatus includes: a reference clock signal generating unit which generates a reference clock signal; a light beam generating unit which generates a plurality of light beams detection signals corresponding to the plurality of light beams respectively; an image clock signal generating unit and the beam detect sensor and responds to the reference clock signal and the plurality of light beam detection signals to successively generate a plurality of image clock signals of the same period corresponding respectively to the plurality of light beam detection signals; a modulating unit which is connected to the image clock signal generating unit and which responds to the plurality of image clock signals respectively to modulate the plurality of light beams generated by the light beam generating unit based on a plurality of image data respectively; a driving circuit which is connected to the light beam generating unit and the modulating unit and drives the light beam generating unit based on the plurality of modulated light beams; a light beam scanning unit which performs periodic scan on a photoreceptor with the plurality of light beams generated by the light beam generating unit; and an image recording unit which records an image based on the plurality of light beams with which the scan is performed on the photoreceptor.
When a plurality of light beams are detected by the beam detect sensor, the image clock signal generating unit produces image clock signals each having the same period each after the same time has passed for each of light beam detection signals. In response to the image clock signals, light beams are modulated and the modulated light beams are generated by the light beam generating unit. In this way, the scan starting positions on the photoreceptor with the light beams generated by the light beam generating unit match each other, thus deviation between a plurality of lines of an image can be reduced. With a simple structure, variation in intensity of the image recorded on the photoreceptor can closely be reproduced, and an even image with excellent gradations can be obtained.
Preferably, the reference clock signal generating unit is a single reference clock signal generating unit.
The image clock signal generating unit successively generates a plurality of image clock signals based on a single reference clock signal.
Therefore, even if the image clock signals are produced by image clock signal generating units, the image clock signals can be produced under the same condition. Deviation between a plurality of image lines can thus be reduced. In this way, variation in intensity of an image recorded on the photoreceptor can closely be reproduced and thus an even image having excellent gradations can be obtained.
Still preferably, the beam detect sensor outputs a plurality of light beam detection signals individually.
A plurality of light beam detection signals can be obtained from one beam detect sensor without using a plurality of light beam detect sensors. Positional adjustment as required when a plurality of beam detect sensors are used is unnecessary. A plurality of precise light beam detection signals can thus be obtained and deviation between a plurality of image lines can be decreased. Consequently, variation in intensity of an image recorded on the photoreceptor can closely be reproduced and thus an even image with excellent gradations can be obtained.
Still preferably, the light beam generating unit includes a plurality of light emitting sections.
The positions of the plurality of light emitting sections are displaced from each other in order to prevent a plurality of light beams from simultaneously entering the beam detect sensor. In this way, a plurality of light beams can be detected by one beam detect sensor.
Still preferably, the modulating unit outputs, after counting the plurality of image clock signals respectively a predetermined number of times, the plurality of modulated light beams.
The modulated light beam is output after the image clock signal is counted a predetermined number of times. During this period, the light beam can reach the image scan starting position on the photoreceptor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.